Quantized feedback may be used in control loops to perform analog to digital conversion. Analog to digital converters (ADCs) with such features are often known as sigma-delta (ΣΔ) converters, or ΣΔ modulators, the modulator term referring to an output digital data stream having a certain symbol pattern, or modulation, imposed upon it by the control loop. The terms ΣΔ modulator and noise shaping control loop are often used interchangeably in the art, although the latter is more descriptive. Circuit designers often like to use such ΣΔ modulators as in many cases they may be simpler to design and cheaper to make than other types of ADCs.
In such a noise shaping control loop, a continuous analog signal is applied at the input, and a digital pattern representative of this signal emerges from the output. The digital signal is created by one or more quantization elements in the control loop, for example, by non-linear elements in the loop such as flip-flops or comparators that have a discrete set of non-continuous output values for any given continuous input quantity.
The ΣΔ modulation works by constraining a feedback parameter to one of a set of at least two specific values, and a control loop of arbitrary order ensures that the average feedback value equals the input. Instantaneous deviations from the ideal continuous feedback necessarily introduced by quantization elements represent noise, and a sophisticated, possibly high order, control loop can suppress or “shape” this noise. To “shape” the noise means to filter it, generally to make it not appear in certain frequency bands. The loop therefore operates to suppress this noise in certain frequency bands of interest, often at the expense of increased noise in bands that are not relevant to the application. Hence ΣΔ modulators are sometimes also referred to a “noise shaping loops.”
Single-bit noise shaping loops are uncommon, due to a known problem in a quantized feedback loop of the type that integrates the feedback signal, i.e., a control loop of a continuous time design (rather than a switched capacitor design) in which the average value of the feedback signal is the time integral of that signal; this problem is known as intersymbol interference (“ISI”). ISI is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the interference from the previous symbols has a similar effect to noise, and make the communication less reliable. The spreading of the pulse beyond its allotted time interval causes it to interfere with neighboring pulses.
The details of the transition between feedback levels are important in integrating feedback loops. FIG. 1 is an illustration of how mismatched transition times can cause ISI. FIG. 1 illustrates how the transition between two feedback levels, a “high” level and a “low” level, here shown as 1 and 0, can be matched or mismatched. The upper curve in FIG. 1 shows matched rise and fall times, i.e., the transition time from 0 to 1 and the transition time from 1 to 0 take the same amount of time, while the lower curve shows transition times that are mismatched, i.e., they differ. This mismatch of rise and fall times introduces a signal dependent error due to a variation of the number of rise and fall edges with the signal amplitude which will cause ISI, and therefore an error will be present in the output.
It is thus apparent that ISI may be suppressed based on the observation that if the number of edges present in the feedback signal were constant, then the ISI from mismatched rise and fall times would represent only a DC shift in the transfer characteristic and no noise or distortion would arise. One of skill in the art will be able to find prior art attempts to suppress ISI by forcing a constant frequency of signal edges in the feedback signal.
However, this known solution has a significant drawback. When a constant frequency of signal edges is present in the feedback, the dynamic range is limited because the modulator cannot produce a constant full-scale feedback pattern. Were the modulator to produce such a pattern, i.e., one in which the quantizer produces all high level or low level outputs, there would be no changes, i.e., no signal edges, in the feedback. Consequently, schemes to force a fixed frequency of feedback edges result in a reduction of dynamic range.
Other known solutions seek to limit the effect of ISI by increasing the number of bits in the noise shaping loop. However, such solutions involve additional components, and thus expense, in constructing the ΣΔ modulator.
It is thus desirable to find a method to reduce ISI in a single-bit ΣΔ modulator without reducing the dynamic range of the ΣΔ modulator.